1. Field of the Invention
The present invention relates to isolated polypeptides having expansin activity and isolated polynucleotides encoding the polypeptides. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods of producing and using the polypeptides.
2. Description of the Related Art
Cellulose is a polymer of the simple sugar glucose linked by beta-1,4-bonds. Many microorganisms produce enzymes that hydrolyze beta-linked glucans. These enzymes include endoglucanases, cellobiohydrolases, and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
Expansins are reported to function in plants in cell growth, cell wall disassembly, and cell separation (Cosgrove, 2000, Nature 407: 321-326). In particular, expansins influence cross-linking relationships in the plant cell wall and allow cell wall components to “slip” during fiber expansion, thereby allowing the fibers to increase in length. There are two classes of expansins that are currently recognized, alpha-expansins (EXPA) and beta-expansins (EXPB) (Sampedro and Cosgrove, 2005, Genome Biol. 6: 242). Alpha-expansins are a conserved group of proteins that function in cell wall enlargement and possibly other developmental processes including cell wall disassembly and cell separation. Beta-expansins have cell wall loosening effects on grass cell walls. It appears that one role of beta-expansins is to soften the stigma and stylar tissues to speed penetration of the pollen tubes through the maternal tissues to the ovule. Other beta-expansins are found in young grass seedlings and non-pollen tissue, thus, suggesting a variety of developmental functions of these proteins.
The plant cell wall is quite high in tensile strength, and must be loosened to enable the cell to grow and expand, thus enlarging irreversibly. Within the cell wall, this expansion of surface area involves slippage or movement of cellulose microfibrils, which normally is coupled to simultaneous uptake of water. In physical terms, this mode of wall expansion requires cell turgor pressure to stretch the cell wall and put the network of interlinked cellulose microfibrils under tension. By loosening the linkages between cellulose microfibrils, expansins allow the wall to yield to the tensile stresses created in the wall through turgor pressure (Cosgrove, 2005, Nat. Rev. Mol. Cell Biol. 6: 850-861).
Expansin reportedly disrupts the non-covalent adhesion or entrapment of hemicellulose on the surface of cellulose microfibrils. Hemicellulose can tether cellulose microfibrils together, forming a strong load-bearing network. Expansin is thought to disrupt the cellulose-hemicellulose association transiently, allowing slippage or movement of cell wall polymers before association reforms and the integrity of the cell wall network is reestablished (Yennawar et al., 2006, Proc. Nat'l. Acad. Sci. 103: 14664-14671).
It would be advantageous in the art to improve the ability to enzymatically degrade or convert lignocellulosic feedstocks.
The present invention provides polypeptides having expansin activity and polynucleotides encoding the polypeptides.